A thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. Thin film transistors manufactured using amorphous silicon have low field effect mobility, but can be formed over a glass substrate with a larger area. On the other hand, thin film transistors manufactured using polycrystalline silicon have high field effect mobility, but a crystallization step such as laser annealing is necessary and are not always suitable for a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique for manufacturing a thin film transistor using an oxide semiconductor, and applying the transistor to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using an oxide semiconductor containing zinc oxide (ZnO) or containing indium, gallium, and zinc as an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.